An amateur scientist discovers the secret of immorality but has a stroke before he can use the elixir himself. As he dies, he gives instructions for how to mix the two chemicals involved and says they must be mixed at forty degrees mumble-mumble or it won’t work—but whether the mumble-mumble is “Celsius” or “Fahrenheit,” he feels no need to explain.

Well, I’ll give this one a certain number of points because I should have been able to figure it out myself. It doesn’t matter whether it’s Celsius or Fahrenheit, because the mumble-mumble is “below zero,” and the two temperature scales have the same -40 mark.